The present invention relates to a novel glycerol derivative having special characteristics.
It has generally been known that when amphipathic molecules such as phospholipids are dispersed in water, they preferentially form molecular aggregates in a special shape. Among these, liposome is a closed endoplasmic reticulum formed from a bilayer of lipid and contains aqueous layer therein. For this reason, it has attracted special interest recently in the field of medicine and pharmacy and accordingly, many attempts have been directed to the use thereof as carriers for various medicines or as diagnostic agents through incorporation of water-soluble substances into the liposome (see, for instance, SUNAMOTO et al., Bioscience and Industry, 1989, 47, p. 475). In addition, attempts have also been directed to the use thereof in cosmetics while making use of the water retention and moisture retention characteristics of the liposome.
When phospholipids are dispersed in water to use them in the form of liposomes or emulsions, it is important that they can easily be dispersed in a certain medium to give a uniform dispersion and that the resulting dispersion surely has good stability. Therefore, it is a matter of course that materials having good dispersibility and stability should be selected and used in such applications. In case of liposome, for instance, the flowability of the liposome bilayer is greatly affected by temperature and it causes gel-liquid crystal phase transition. It has been known that the mobility of molecules present in a bilayer which is in a liquid crystalline state is substantially higher than that observed for molecules in a bilayer which is in a gel state with respect to all of lateral diffusion, flip-flop and exchange. In general, liposomes formed from lipids whose hydrophobic fatty acid residues have small numbers of carbon atoms and/or high degrees of unsaturation have high membrane flowability and on the contrary, those formed from lipids which are saturated and have relatively large numbers of carbon atoms have low flowability and rather high phase transition temperatures. Thus, it would be possible to control the membrane flowability of the liposome and the dispersibility of lipids as well as barrier properties of the membrane closely related to the flowability by adjusting the chain length and the degree of unsaturation of the fatty acid residues of lipids used.
For instance, lipids which comprise, as components, unsaturated fatty acids such as yolk phosphatidylcholine are in liquid crystal states at a temperature of not less than ordinary temperature because of low phase transition temperatures and accordingly form soft membranes. The lipids having unsaturated fatty acids are indispensable to living body since biological membranes in the liquid crystalline state serve as barriers. It can be assumed that the membranes of the organism would acquire the characteristics of this kind because it is advantageous that the membranes cause change in physical properties for relieving abrupt changes in various environmental conditions such as ambient temperature. This phenomenon is likewise important when the liposome is used as a carrier for medicines. For instance, when a hydrophobic medicine is incorporated into a liposome membrane, good dispersibility and high encapsulation efficiency can often be attained by the use of those comprising unsaturated fatty acids such as yolk phosphatidylcholine as compared with those simply comprising saturated fatty acids.
However, the polyvalent unsaturated fatty acids included in the yolk phosphatidylcholine are liable to undergo a peroxidation reaction with oxygen and thus are insufficient in the storage stability. For this reason, it is preferred to use phospholipids simply comprising saturated fatty acids which are hardly attacked by oxygen if the stability is regarded as of major importance. However, when a liposome is prepared using dimyrystoyl phosphatidylcholine which is a naturally occurring saturated phospholipid as a membrane component, it is very difficult to hold glucose within the resulting liposome at the phase transition temperature thereof or higher. In addition, it has likewise been known that the liposome prepared from palmitoyl phosphatidylcholine alone is unstable and immediately causes aggregation and correspondingly precipitation. In general, the phospholipids comprising only saturated fatty acids are densely arranged, in particular at the phase transition temperature or lower and have low flexibility. As a result, they have a strong tendency such that they exclude foreign molecules and cause phase separation. Therefore, it is impracticable to prepare liposomes from the phospholipids simply comprising saturated fatty acids.
As has been discussed above, good dispersibility and good flowability of the liposomes are essentially contrary to the chemical stability thereof and it is impossible to simultaneously satisfy these three requirements. In other words, there have not yet been known any materials inclusive of the conventionally used phospholipids simply comprising saturated fatty acids and the phospholipids comprising unsaturated fatty acids which can satisfy these requirements at the same time.
Under such circumstances, the inventors of this invention have conducted various investigations for searching for substances which satisfy all of the foregoing requirements and as a result, the inventors have taken note of biomembranes of bacteria. In general, bacteria do not contain any polyvalent unsaturated fatty acid in the biomembranes unlike animals and plants. About 30 years ago, investigators in Japan discovered, for the first time, the presence of branched fatty acids (such as iso acid and anti-iso acid) as principal fatty acids for bacterial lipids (S. Akashi and K. Saito, J. Biochem., 1960, 47, p. 222). There have presently been known almost several hundred kinds of bacteria which possess such branched fatty acids as principal fatty acids in biological lipids (T. KANEDA, Bacteriol. Rev., 1977, 41, p. 391).
Recently, many diacyl phosphatidylcholines comprising various branched fatty acids have been synthesized using the structure of the foregoing branched fatty acid as a model and the phase transition temperatures thereof were determined. As a result, it is proved that the phase transition temperatures of lipids comprising branched fatty acids are about 16.degree. to 28.degree. C. lower than those of lipids comprising linear fatty acids. In other words, the branched fatty acids contribute to an increase in the flowability of bacterial biomembranes as compared with corresponding linear fatty acids (T. KANEDA, Bioscience and Industry, 1990, 48, p. 229). These branched fatty acids are considered to be desirable substances since they are hardly attacked by oxygen unlike the polyvalent unsaturated fatty acids and have high chemical stability. However, iso acid and anti-iso acid are not universarily present in the natural world and it is very difficult to purify and/or isolate them since they are present in the form of a mixture with other substances carrying hydrophobic moieties of various structures. Thus, the only means left is chemical synthesis thereof, but there is a limit in raw materials easily available. Moreover, the mass-production thereof is very difficult since a lot of processes are required for extending the carbon chain length.
To solve these problems, the biomembranes of archaebacteria have recently attracted special interest. The term "archaebacteria" is a recent concept proposed by Woese et al. in 1977 on the basis of the comparison between base sequences of 16s rRNA's of various organisms and there have presently been known three groups, i.e., highly halophilic bacteria, sulfur-dependent highly thermophilic bacteria and methane-producing bacteria (see Yosuke KOGA, "Archaebacteria", 1988, published by Tokyo University Publishing Society). All of the polar lipids of archaebacteria presently known are glycerolipids having ether bonds and most specific property thereof is that the hydrocarbon chains thereof are saturated isoprenoids having 20 or 40 carbon atoms. The saturated isoprenoids are likewise hardly attacked by oxygen and chemically stable. Thus, it can be anticipated that materials having special properties can be obtained if artificial lipids are designed and synthesized while using these lipids as models.
Chain isoprenoids are relatively easily available as compared with iso acid and anti-iso acid and accordingly, there have recently been reported various experimental results of lipids which have these isoprenoids introduced into the hydrophobic moieties thereof (K. Yamauchi et al., Biochem. Biophys. Acta, 1989, 1003, p. 151; K. Yamauchi et al., J. Am. Chem. Soc., 1990, 112, p. 3188; L. C. Stewart et al., Chem. Phys. Lipids, 1990,54, p. 115; YAMAUCHI et al., Collected Resume of Heisei 2 (1990) Spring Annual Meeting of Chemical Society of Japan, pp. 1793, 1794; TODA et al., Collected Resume of Heisei 2 (1990) Spring Annual Meeting of Chemical Society of Japan, p. 1793; R. A. Moss et al., Tetrahedron Lett., 1990, 31, p. 7559; and Japanese Unexamined Patent Publication (hereinafter referred to as "J. P. KOKAI") No. Hei 2-288849). It has consequently been found out that the lipid bilayers formed from isoprenoid type lipids have not only low phase transition temperatures but also high membrane barrier properties. However, in the molecular design of isoprenoid type artificial glycerolipids presently proposed, the glycerol moieties are linked to the hydrocarbon chain moieties through ether bonds similar to the biomembranes of archaebacteria. Therefore, the synthetic methods cannot widely be used because they are inconvenient for the mass-production thereof.
Further, there has been known an example in which a quaternary ammonium salt having an isoprenoid chain in lipid bilayers and lipid bilayers immobilized onto polymer films (see J. P. KOKAI No. Hei 2-288849). As a result, there have been found out that these lipid bilayers show flowability approximately identical to that of the biomembranes and that the film carrying the lipid bilayers immobilized thereon are soft and flexible and have excellent strength.
As has been discussed above in detail, all of the conventionally known phospholipids inclusive of those comprising only saturated fatty acids and those comprising unsaturated fatty acids do not satisfy all of the foregoing requirements simultaneously. It may be anticipated that lipids having branched fatty acids or chain isoprenoid skeletons, in particular, iso acid or anti-iso acid would exhibit promising characteristics, but it has been very difficult to isolate and purify them from the natural resorces and to chemically synthesize the same.